It is absolutely essential for the position of the substrate to be precisely related to the position of the mask, particularly when the substrate has already been provided with circuit components or patterns, by means of one or more preceding exposure operations. In these circumstances, it is not sufficient for the substrate to be moved into a position defined by abutment means, and to be moved by predetermined increments, but, it is necessary to provide for direct monitoring of the position which has actually been reached, and to provide the possibility of suitable correction. This monitoring operation is effected by images of adjustment marks on the substrate and an alignment pattern on the mask formed in the same plane, and by checking the relative association thereof. Usually, this image projection operation is effected in the plane of the projection mask. Because of the reversability of the optical beam, forming an image of the alignment pattern on the mask in the plane of the substrate, in which the comparison of positions required for the adjustment operation can also be effected, will always correspond to the above-mentioned formation of images of the alignment pattern and the adjustment marks in the same plane.
If, in an arrangement of this kind, the adjustment operation is carried out with the same wavelength as the exposure operation, correction in respect of the projection lens for this wavelength also provides the necessary accuracy in forming the image of the mask of the substrate and the adjustment mark regions of the mask and the substrate in each other.
However, the amount of light which falls onto the light-sensitive layer during the adjustment operation may not exceed a given value (about 1% of the exposure light). This process is known for example from DOS No. 27 07 477 or DOS No. 25 39 206, page 6, paragraph 3. A disadvantage with this process is the weakness of the signal which is available for the adjustment operation and which increases the difficulty of and the time required for the adjusting operation. Usually therefore, adjustment on the one hand and subsequent exposure on the other hand are effected by two different wavelengths of light, the adjustment wavelength and the exposure wavelength, so that there is no erratic preliminary exposure of the photosensitive lacquer which is sensitive only to the exposure wavelength, during the adjustment operation.
Moreover, the projection lens must project the mask sharply onto the substrate, at the exposure wavelength and, at the adjustment wavelength, the projection lens must project the alignment pattern of the mask sharply onto the adjustment mark region of the substrate and/or vice-versa. This would be readily possible with a projection lens which is corrected both at the adjustment wavelength and also the exposure wavelength. In that case, the plane of the mask and the plane of the substrate, in which planes the alignment patterns and the adjustment marks respectively lie, are combined together at both wavelengths and are identical for both wavelengths. For this reason, the notional separation between forming the image of the actual mask and of its alignment pattern is also unnecessary in this respect, and is also not usually done. The main disadvantage of this arrangement was that a diffraction-limited higher-power lens with a very high numerical aperture and a large image field could hitherto only be corrected at one wavelength and can therefore not be used in this process.
In a piece of equipment which was produced some time ago, an auxiliary lens was added to the equipment for the purposes of adjustment of the projection lens proper, the auxiliary lens causing correction at the adjustment wavelength. The disadvantage of this process is that the auxiliary lens must be pivoted into and out of the path of the beam, and this causes time to be lost, gives rise to vibration and requires an expensive mechanism.